Personally I don't think so. I have just replaced my Geforce 9600GS with a 520 GT. I had frame dropping on high bitrate H264 encoded movies. I also noticed a slight stutter every now and then even though XBMC did not detect frame dropping. The motion during panning was just not smooth.

I took a bit of a gamble there when I put in a new GPU, but then a Geforce 520 GT is not that expensive. The result for the time being looks great, tonight I will watch my first full movie on 1080p since I put in the new card last Monday.

The advantage of a new GPU is also that you can bitstream HD audio codecs which is really the future.